Phosphorus is an important element, and indeed essential to life. However, the release of phosphorous to surface waters, and its consequent contribution to eutrophication, has also led to increasing concerns about water quality. Policies were therefore implemented throughout the world, to reduce the levels of phosphorus entering surface waters, by the implementation of technologies to remove phosphorus from domestic and industrial wastewater. As a consequence, phosphorus accumulates in sewage sludge which is a major by-product of wastewater treatment plants.
Mineral phosphorus resources are considered limited and finite. Therefore, there is an increasing interest for technologies that can facilitate the recycling and beneficial re-use of the phosphorus present in wastes such as sewage sludge.
Fertilization with sewage sludge has decreased in an increasing number of countries due to difficulties of finding agricultural land around big cities that can accept large volumes of sludge but also due to concern regarding contents of pollutants such as heavy metals, organic contaminants, and pathogens in sewage sludge. Incineration is practiced as a solution to reduce the volume of sewage sludge and to destroy organic contaminants and pathogens before disposal.
Dewatered sewage sludge still contain considerable amount of water (about 70-80 percent), a main part as intracellular water in micro-organisms. Therefore, sewage sludge has to be dried to about 40 percent dry matter in order to enable incineration. Dedicated plants for mono-incineration of sewage sludge have been built in several countries. In these plants, the incoming sewage sludge is dried with recycled heat prior to the incineration.
Ash of mono-incinerated sewage sludge contains about 6-14 percent phosphorus by weight, which is slightly lower than the concentration of phosphorus in phosphate rock (e.g. 12-16 percent phosphorus by weight) which is the typical raw material for production of inorganic phosphorus fertilizers. More than 90 percent of the phosphorus being present in the sewage water is also found in the ash. The phosphorus present in the ash is insoluble in water due to bindings with calcium, iron or aluminium. Therefore, the phosphorus-fertilizer value of ash is low. Furthermore, heavy metals are enriched in ash and hinder the direct recirculation of ash to cropped land.
In addition to mono-incineration, sewage sludge can also be co-incinerated with biomass such as wood chips, etc. The advantage of such an approach is that dedicated plants for incineration of sewage sludge are not required. A mixture of sewage sludge and biomass can be incinerated in conventional plants for waste incineration without pre-drying of the mixture. However, the consequence of co-incinerating sewage sludge with biomass is that the phosphorus concentration in the ash is reduced, commonly to below 5 percent.
A number of methods have been developed to recover phosphorus from mono-incinerated sewage sludge, as its phosphorus content is high.
Japanese patent 9145038 describes a process based on heating the ash up to 1,400° C. to vaporize elemental phosphorus, which is condensed in water and oxidized to phosphoric acid. The drawbacks of this approach are that heating sludge ash to evaporate phosphorus requires large amounts of energy and the efficiency of phosphorus recovery is moderate due to formation of iron phosphate slag.
The published European patent application EP2016203 (A1) describes a process for thermochemical removal of heavy metals from sludge ash. The process is based on addition of earth metal chlorides to the ash and heating to above 900° C. to evaporate heavy metal chlorides. The drawbacks of this approach are large energy requirement for heating, phosphorus in the residue remains water-insoluble (low fertilizer value), and the phosphorus concentration in the residue is reduced due to dilution with elements remaining in the ash and with added chemicals. Processing of co-incinerated sludge ash which originally has a relatively low phosphorus concentration will probably result in too low phosphorus content in the product.
The published international patent application WO 00/50343 describes a process for recovering iron, aluminium and phosphorus from ash leach solution using ion exchange. The drawbacks of the process include high costs due to the need for large excess of regeneration chemicals and recovery of solutions with a relatively low concentration.
In the published international patent application WO 2008/115121, a method and an arrangement for phosphorus recovery are disclosed. The method is applicable to recovery of phosphorus from ash leach solutions. Separation of iron and aluminium is performed with a strong cation exchange resin regenerated with a mineral acid. The disadvantages are similar to disclosure WO 00/50343 and include high costs due to the need for a large excess of regeneration chemicals, limited value of recovered iron and aluminium products due to contamination with acid, low concentration, and that it is not possible to recover iron and aluminium separately.
Schaum et al. described in a conference (Conference on the Management of Residues Emanating from Water and Wastewater Treatment, Dec. 8, 2005, Johannesburg, South-Africa) a process for phosphorus recovery from sludge ash. The process is based on dissolution of mono-incinerated sludge ash in sulphuric acid followed by addition of sodium hydroxide to the leach solution to precipitate a product composed of mainly aluminium phosphate. The drawbacks of the process include a high cost due to the use of expensive sodium hydroxide and limited value for the recovered aluminium phosphate product. Aluminium phosphate has a very low solubility in water and thus cannot release phosphorus at rates sufficient for crops when used as a fertilizer. Furthermore, aluminium is toxic for plants. The fertilizer value of aluminium phosphate is therefore very low.
Franz described in a scientific publication (Waste Manag. 2008; 28(10):1809-18) a process for phosphorus recovery based on dissolution of sludge ash in sulphuric acid followed by precipitation of phosphorus from the leach solution with lime. The drawbacks of the process include low P content in the recovered product due to dilution with formed gypsum, considerable amounts of toxic aluminium in the product, and low plant availability of phosphorus due to binding in a water-insoluble form. The fertilizer value of the product is therefore very low.
Dittrich et al. described in a conference (International Conference on Nutrient Recovery from Wastewater Streams, Vancouver, 2009) a process for phosphorus recovery from sludge ash based on dissolution of sludge ash in hydrochloric acid followed by extraction of iron and heavy metals with alamine 336 solvent and thereafter precipitation of phosphorus in form of aluminium phosphate and calcium phosphate with lime. The drawbacks of the process include a higher cost for hydrochloric acid compared to sulphuric acid, regeneration of the alamine 336 solvent requires use of both ammonium carbonate and hydrochloric acid which is costly, production of large quantities of precipitated iron together with heavy metals which has to be disposed, recovery of phosphorus as a mixture of water-insoluble calcium phosphate with aluminium phosphate having a low fertilizer value.
The published international patent application WO 03000620 describes a process for treatment of sludge comprising the steps of putting the sludge into conditions being supercritical for water, adding an oxidant particularly oxygen to the sludge, separating the phosphorus from water and from carbon dioxide formed during the oxidation and recovering phosphorus by means of dissolving phosphorus in alkali. The main drawback of the process is the requirement of treating sludge by supercritical water oxidation which is complex. If the principles of dissolving phosphorus in alkali are applied to sludge ash, the recovery rate is very low, commonly below ten percent.
There is a need for a method for phosphorus recovery from materials containing phosphorous and at least one of iron and aluminium, e.g. sludge ash, in which a major part of the phosphorus can be recovered in a valuable form, clean from heavy metals, which can be used to produce fertilizers with high plant availability or as feed phosphates. Furthermore, the method should enable processing of any type of sludge ash independent on the precipitation chemical used in the wastewater treatment plant. Phosphorus recovery should be cost efficient and enable processing of ashes with low phosphorus content such as co-incinerated sludge ash. Furthermore, separation and recovery of other elements present in ash such as calcium, iron, aluminium, heavy metals, etc. is desired in order to increase recirculation of elements in society and to reduce the need for disposal of ash residues.